fix(chat): tie conversation cursors to the Codex rollout chain

[Yoonwoo-Ha]

Follow-up to #5, as requested in its merge review.

Cursors are tied to the earlier-rollout chain

A cursor's id named only the live file (dev-ino), while its offset counts from the start of the whole Codex chain. If the chain changed between requests (an earlier rollout found later, or a cached chain evicted after its parent was archived), an old cursor pointed at other turns. before returned a page from elsewhere, and the gap fill showed turns twice and out of order. It never returned a 409.

The id now carries a fingerprint of the earlier rollouts: a hash of each one's path, the bytes of it the chain uses, and its file identity (dev:ino). A cursor from a different chain fails parseCursor, answers 409 history_changed, and the chat reloads.

• The stream is identified before the page cache is consulted, so a cached older page is keyed by the chain as well.
• A single-file conversation keeps its old id format (no suffix), so existing cursors stay valid across the upgrade.

Unresolved chains are remembered briefly

A chain that stops at a cut that no file holds used to walk sessions/ again on every append. It is now cached for 30s (complete chains stay cached as before), so a rollout that shows up later is still picked up.

Tests

• conversation.test.ts: a cursor issued before the earlier rollout appears answers HistoryChanged afterwards, for both before and from, and a fresh read then pages back to the earlier rollout's first prompt. Without the fingerprint the test fails: the old cursor returns a page from the wrong place.
• codex.test.ts: the "shows less history" case clears the chain cache (forgetHistoryChains) before it expects the late rollout to resolve.
• bun test: 269 pass, 0 fail. Typecheck clean.

🤖 Generated with Claude Code

[devswha]

Merged. Thanks. The chain fingerprint covers what can change, and the new test fails without it.

One follow-up for a separate PR: a cached chain doesn't recover when its parent is archived. historyChain (server/codex.ts) keeps a complete chain until 64 others push it out. If the parent rollout is archived during that time, every read that reaches the parent fails with transcript_missing, and the route answers scrollback. Right after a backtrack the live file is small, so even the newest page reaches the parent, and the whole chat falls back to terminal output until the entry is pushed out or the server restarts. Suggested fix: forget the cached chain when an earlier segment fails to open. The chain is then looked up again, comes back shorter because archived_sessions/ isn't searched, and gives one 409, which is the right outcome.